At the present time, development and study on various display devices are aggressively driven. In particular, organic EL devices can obtain highly bright luminescence at a low voltage and accordingly, are drawing an attention as a promising display device. For example, an EL device comprising an organic thin film formed by depositing an organic compound is known (see, Applied Physics Letters, Vol. 51, page 913 (1987)). The organic EL device described in this publication has a laminate structure comprising an electron transporting material and a hole transporting material and is greatly improved in the luminescence properties as compared with conventional single-layer devices.
As a means to improve the luminescence efficiency of the laminate-type EL device, a method of doping a fluorescent dye is known. For example, an organic EL device having doped therein a coumarin dye described in Journal of Applied Physics, Vol. 65, page 3,610 (1989) is greatly improved in the luminescence efficiency as compared with non-doped devices. In this case, by changing the kind of the fluorescent compound, light having a desired wavelength can be taken out but compounds capable of highly efficiently emitting blue light and having superior durability are small in number.
Examples of the blue light emissive material for organic EL devices include condensed polycyclic aromatic compounds such as anthracene, pyrene and perylene (compounds described, for example, in J. Chem. Phys., 44, 2902 (1966), and Thin Solid Films, 99, 171 (1982)), tetraphenylbutadiene-based compounds (compounds described, for example, in JP-A-59-194393 (the term "JP-A" as used herein means an "unexamined published Japanese patent application")), distyrylbenzene-based compounds (compounds described, for example, in European Patents 281381, 319881 and 373582, U.S. Pat. Nos. 4,672,265, 4,725,531, 4,734,338, 4,741,976 and 4,776,320, JP-A-61-37890, JP-A-1-245087, JP-A-2-247277, JP-A-2-247278, JP-A-2-209988, JP-A-3-33184, JP-A-3-84089, JP-A-3-231970, JP-A-4-117485, JP-A-4-275268, JP-A-5-17765 and JP-A-140145), stilbene-based compounds (compounds described, for example, in JP-A-2-235983 and JP-A-3-47890), polyphenyl-based compounds (compounds described, for example, in JP-A-3-33183), polycarbonate-based compounds containing an aromatic tertiary amine skeleton as a repeating unit (compounds described, for example, in JP-A-5-247459), metal complexes having a tropolone skeleton (compounds described, for example, in JP-A-9-157642), and metal complexes having a 2-(o-hydroxyphenyl)benzoxazole derivative or 2-(o-hydroxyphenyl)-thiazole derivative as a ligand (compounds described, for example, in JP-A-7-133483, JP-A-8-113576, JP-A-8-301877 and JP-A-8-306489). However, these compounds are disadvantageous in that the luminescence efficiency is low, the durability has a problem and the color purity is bad, thus, cannot be used in practice.
Of organic EL devices, devices in which an organic substance is laminated by vacuum deposition can successfully achieve highly bright luminescence, however, in view of simplification of the production process, workability and the like or for obtaining a large area, the device is preferably fabricated by a coating method. The devices fabricated by the conventional coating method are, however, inferior in the luminance of light emitted and the luminescence efficiency to the devices fabricated by the deposition method. Thus, improvements are still in need for attaining high luminance and highly efficient luminescence.